March 7, 2014

Pine Tree Branches Turned Into Effective Water Filtration Systems

The next time you find yourself lost in the woods with no clean drinking water, the nearest pine tree may save your life. While lake or pond water may provide some short-term relief from dehydration when in the wild, these sources of water are not always clean.

This is where the pine tree comes into play. Pouring lake water through a freshly-peeled pine tree limb can effectively remove most bacteria that may exist in the water, leaving you with a clean and fresh source of H2O.

This process is so effective that MIT researchers, publishing a paper in the journal PLOS ONE on Feb. 26, found that the low-tech filtration system can produce up to four liters of clean drinking water per day.

The research team demonstrated that a small piece of sapwood can filter out 99 percent of E. coli bacteria from water. This sapwood, which contains xylem tissue that helps transport sap up through the tree, has pores that allow water to pass through but trap most bacteria from filtering through.

Rohit Karnik, an associate professor of mechanical engineering at MIT, says that this sapwood is a low-cost efficient material for filtering water and could go a long way in helping rural communities where advanced filtration systems may not be accessible.

“Today’s filtration membranes have nanoscale pores that are not something you can manufacture in a garage very easily,” said Karnik, one of the study coauthors. “The idea here is that we don’t need to fabricate a membrane, because it’s easily available. You can just take a piece of wood and make a filter out of it.”

This sapwood system also doesn’t have the drawbacks that come with other more advanced filtration systems. Chlorine-based filters work well but can be expensive; boiling water also relies on costly fuels to heat the water; membrane-based filters are also expensive and require a pump and easily become clogged.

The xylem network in pine trees consists of a system of vessels and pores, helping sap move from the roots to the crown of the tree. Each vessel wall is pockmarked with tiny pores called pit membranes, through which sap can flow from one vessel to the next as it feeds the tree. These pores also limit cavitation – a process by which air bubbles can grow and spread in the xylem, leading to tree death. The xylem’s pores actually trap bubbles, preventing them from spreading throughout the tree.

“Plants have had to figure out how to filter out bubbles but allow easy flow of sap,” Karnik noted. “It’s the same problem with water filtration where we want to filter out microbes but maintain a high flow rate. So it’s a nice coincidence that the problems are similar.”

For the study, the team collected branches of white pine and stripped off the outer bark. They cut small sections of sapwood measuring about an inch long and half-inch wide and then mounted each in plastic tubing, sealed with epoxy and secured with clamps.

The team first used water dyed with red ink particles ranging from 70 to 500 nanometers in size. Once the water had filtered through the system, the team cut open the sapwood filter lengthwise and observed that much of the red dye was contained within the very top layers of the wood. The filtered water, which passed through easily, was free of any red dyes and crystal clear. This experiment proved that sapwood is naturally able to filter out any particles larger than 70 nanometers in size.

In a different experiment, the team found that sapwood is unable to filter out particles 20 nanometers in size or smaller, suggesting there is a limit to the size of particles that sapwood can naturally filter.

In a third experiment, the team used inactivated E. coli-contaminated water. After pouring the contaminated water through the sapwood filtration system, the team examined the xylem under a fluorescent microscope, noticing that the bacteria had accumulated around pit membranes within the first few millimeters of the wood. Through their calculations, they determined that the sapwood was able to filter out more than 99 percent of the E. coli from the water.

Based on these findings, Karnik believes that sapwood can filter out most types of bacteria – the smallest bacteria measure around 200 nanometers. He said it is unlikely that the filtration technique cannot trap most viruses, as most of these are much smaller in size.

He said his team now plans to experiment with other types of sapwood to see if nature’s filtration system exists in other trees as well. Flowering trees typically have smaller pores than coniferous trees, suggesting they may be able to filter out smaller bacteria and possibly viruses. However, the vessels in flowering trees are much longer, which could ruin their chances of being feasible filtration systems.

Another key issue is that the sapwood would need to remain damp in order to be used effectively. Once the sapwood dries, it cracks and cannot properly filter contaminants from the water.

“There’s huge variation between plants,” Karnik said in a statement. “There could be much better plants out there that are suitable for this process. Ideally, a filter would be a thin slice of wood you could use for a few days, then throw it away and replace at almost no cost. It’s orders of magnitude cheaper than the high-end membranes on the market today.”

Karnik’s research was funded by the James H. Ferry Jr. Fund for Innovation in Research Education. His coauthors include Michael Boutilier and Jongho Lee from MIT, Valerie Chambers from Fletcher-Maynard Academy in Cambridge, Mass., and Varsha Venkatesh from Jericho High School in Jericho, N.Y.